Britain was braced last night for a fresh wave of terrorist attacks as the national threat level was raised to 'critical' following an attempted car bombing of Glasgow airport.

Just four days into his premiership, Gordon Brown was dealing with the most dangerous situation facing Britain since the attacks on London in July 2005. Police and intelligence officers confirmed that there was a direct link between the Scottish attack and the attempted car bombing of London on Friday - confirming the reality of a renewed UK offensive by Islamist extremists.

Last night the Prime Minister summoned intelligence chiefs and ministers before the Cobra emergency committee in Whitehall to discuss the deteriorating security situation. It was agreed to raise the threat level to the highest degree possible, a decision that confirmed another attack is expected imminently.

In a televised address from Downing Street, a sombre-faced Brown urged people to be 'vigilant' and support the police and security services. He said: 'I know that the British people will stand together, united, resolute and strong.'

As night fell, armed police began stopping vehicles entering airports throughout the UK after warnings were circulated that a nationwide terror cell is preparing more attacks. Liverpool John Lennon airport was closed at 8.30pm while police investigated a suspect vehicle. Glasgow airport too remained closed.

At 3.11pm, a Jeep Cherokee wreathed in flames was crashed into the doors of the main terminal building at Glasgow. Driven by two 'Asian-looking' men, it came to a halt as they threw petrol over it and appeared to try to detonate the vehicle. With the help of bystanders, the two men were overpowered and arrested. One was fighting for his life last night, after throwing petrol over himself and setting it alight.

There was a further twist last night as the Royal Alexandra Hospital in Paisley, where the man was being treated, was evacuated. Strathclyde Police later said that the man had possibly been wearing a suicide vest.

William Rae, chief constable of Strathclyde Police, last night confirmed that the attack on Glasgow airport was linked to the London attempt. 'There are similarities between this incident and those in London. We are treating this as a terrorist incident.' The Jeep attack, bearing the hallmarks, police said, of an al-Qaeda plot, came 36 hours after extremists attempted a double car bomb attack in the heart of London's West End.

Two people were arrested in Cheshire late last night in connection with the London and Glasgow failed terrorist attacks, which appear to have slipped completely under the radar of the security services. The pair were arrested by anti-terrorism officers from the Metropolitian and West Midlands Police.

Intelligence sources confirmed the attack on Glasgow airport appeared to be aimed at killing passengers.

As with the failed London attack, the explosives and gas canisters which appeared to be in the Jeep did not detonate, a stroke of fortune that may again have saved the lives of hundreds.

Witnesses described chaotic scenes as the Jeep sped towards the terminal entrance. Robin Patterson, 42, of Rochester, Kent, saw the car burst into flames. 'There was an enormous explosion,' he said. 'The guy next to the car, his skin and clothes just fell off him.' Another described one of the men throwing punches at police while screaming: 'Allah, Allah'.

The Prime Minister's new terrorism adviser, Lord Stevens, said last night: 'Make no mistake, this weekend's bomb attacks signal a major escalation by Islamic terrorists.' Meanwhile in London, a massive investigation into what could have been Britain's most deadly terror attack widened last night. Police and intelligence sources told The Observer that they were now investigating the existence of an Islamic terror cell in the capital. One major branch of the inquiry is tracking down a number of terror suspects who have slipped their control orders, a development that raises fresh questions over their effectiveness.

One man being sought is Lamine Adam, 26, who, in evidence at the recent Crevice terror trial that saw five jailed for plotting fertiliser bomb attacks in the UK, allegedly boasted of targeting nightclubs. One of the Mercedes cars involved in the attack was left outside the busy Tiger Tiger nightclub in Haymarket, in the centre of the West End. The other Mercedes was found in a nearby street.

Other men urgently wanted by police are his brother Ibrahim, 20, and Cerie Bullivant, 24, who have also evaded control orders. The government's independent reviewer of anti-terror legislation, Lord Carlile, has said that there is 'solid evidence' that the trio had wanted to join insurgents abroad and attack British troops serving in Iraq. Security sources said it remains 'possible' that the men were involved in the attempt to bring carnage to London.

After being briefed on the progress of the police investigation, the new Home Secretary, Jacqui Smith, released a statement urging greater vigilance. It read: 'The police are clear that the most important contribution that the public can make is to carry on reporting anything suspicious and to remain vigilant. I must stress we must not let the threat of terror stop us getting on with our lives.'

The mayor of London, Ken Livingstone, reiterated that the capital faced 'a very real threat' and called on Londoners to be 'vigilant'. Hundreds of extra police were drafted on to the streets to step up security and to reassure the public.

The Gay Pride Festival, which saw hundreds of thousands of people making their way from Baker Street to Trafalgar Square, was accompanied by 350 officers, while extra police were drafted in for the Wimbledon tennis championships and the Diana, Princess of Wales, concert at Wembley.

Among other terror suspects police want to track down are former tube worker and 26-year-old Londoner Zeeshan Siddiqui. A court has heard how he trained with a London suicide bomber in Pakistan.

Another individual police want to track down, even if only to eliminate him from their investigation, is Bestun Salim, who disappeared from his Manchester home last year, and is alleged to have links to Ansar al-Islam, a group linked to the terrorist network of the late Abu Musab al-Zarqawi, Iraq's notorious insurgent leader who was killed last year.